Dependency

Go on. Buck the trend. Things like this don't necessarily have a reason other than it being what everybody else does.

You CAN say 'dependence'.
I'm dependent on drugs..
My dependence on heroin..
I don't see anything wrong with that.

The two words are interchangeable and can be used with the same meaning.

Dependency, however, has a special meaning used in Psychology that means overreliance on a person or thing. Strictly speaking, it would be incorrect to say "My dependence on heroin" in the context of psychology. The sentence is correct and the meaning is the same, but it would be counted as incorrect terminology.

Dependency is a specialist word in the psychological sense, just as it is a specialist word when used to mean a subject territory of a state which does not have a contiguous border to that state (such as Hawaii and Alaska).

Don't knock it 'till you've tried it.

You should see the movie or the play "1776", which includes the line (sung), "a declaration of independency..."

Traditionally, a "dependency" was a component that relies on something else, like a commonwealth that depends on a country for defense, while "dependence" was the state of being dependent. I'm not sure when the meanings merged, but it seems like the words are now used interchangeably.

Having said that, "dependency" sort of makes me cringe. It sounds like Stephen Colbert's word "truthiness." I think if you look up the word "dependency" in the dictionary, the definition ought to be "reliantness."

I know someone at work who talks about FCC compliancy when he should say FCC compliance. It makes me cringe. I think some people just think they sound smarter if they use more syllables.